This invention relates to a process for the manufacture of dialdehyde cellulose. It pertains particularly to a process for the manufacture of dialdehyde cellulose on the large commercial scale by the oxidation of cellulosic pulps derived from the papermaking industry.
Dialdehyde cellulose has important real and potential industrial applications as a paper and textile sizing material, as an adhesive component, as a thickening agent for foods, and in the tanning of hides in the manufacture of leather. In particular, it is useful as an intermediate in the manufacture of a variety of other cellulose derivatives, including dicarboxy cellulose, tricarboxy cellulose, glyoxal and the like.
The oxidative conversion to dialdehyde cellulose of polysaccharides in general, and starch in particular, using periodic acid as an oxidizing agent is well known. In the case of cellulose, the reaction proceeds according to the following schematic equation: ##STR1##
The general reaction has been described in Jackson et al, the Journal of the American Chemical Society, Vol. 59, pp. 2049-2050, (1937), and in Pfeifer et al., Industrial and Engineering Chemistry. Vol. 52, pp. 201-207 (1960). It is further characterized in Slager, U.S. Pat. No. 3,086,969. The emphasis in these and other prior art investigations, however, has been toward the oxidative conversion of the polysaccharide starch to dialdehyde starch, rather than the conversion of the polysaccharide cellulose to dialdehyde cellulose.
Thus Jackson et al describe in detail successful conversion of starch to dialdehyde starch, but indicate the resistance of cellulose to the oxidation by citing cellulose oxidation times of from 18 to 37 days, and the degradation of the dialdehyde cellulose product by the acid reaction medium.
Similarly, although Slager discloses in a general way the periodic acid oxidation of cellulose to dialdehyde cellulose, he gives specific examples drawn exclusively to the conversion of starch to dialdehyde starch under carefully defined and critical reaction conditions, in particular using a periodic acid reaction medium having a pH of less than 1.0.
It does not necessarily follow that the convertibility of starch to dialdehyde starch under stipulated reaction conditions is an indication that under the same or similar conditions cellulose can be converted to dialdehyde cellulose. It is well established that each chemical entity has its own peculiar characteristics and properties, not necessarily forecastable by comparison to another chemical entity even though the latter be closely related structurally to the former.
Thus, although starch and cellulose broadly are classified together as polysaccharides, they have in fact significant differences of structure and properties. For example, whereas starch is an alpha glucoside, cellulose is a beta glucoside. Starch is a mixture of polymers of amylose and amylopectin cellulose, a linear polymer of beta glucoses. Starch is a powder; cellulose a fiber. Biologically, starch is digestible by humans, whereas cellulose is non-digestible. Another inherent difference is indicated in the Jackson et al reference, supra, wherein an oxidation time of as little as 24 hours is reported for starch, and an oxidation time of as much as 37 days is reported for cellulose, in the conversion of these two substances to their respective dialdehyde derivatives under substantially similar reaction conditions.
The resistance of cellulose to selective oxidation raises significant obstacles to the application of this procedure to the commerical production of dialdehyde cellulose. Thus, the long reaction times of several or many days are unsuited to the successful operation of a commercial process. The comparative resistance of cellulose to oxidation favors the production of a multiplicity of oxidative byproducts, which not only lower the yield of the desired dialdehyde cellulose, but result in contamination of the product.
The same factor promotes the degradation of the periodic acid oxidizing agent to iodine derivatives other than iodic acid. It promotes notably the production of free iodine which, being per se an active oxidizing agent in aqueous medium, attacks the dialdehyde cellulose and converts it inter alia to carboxy celluloses.
The conversion of periodic acid to iodine derivatives other than iodic acid also destroys one of the principal advantages of the periodic acid system for converting cellulose to dialdehyde cellulose, in that whereas in that system the iodic acid product may be converted electrolytically almost quantitatively back to periodic acid, which then may be reused, that is not the case with free iodine and the other reduced products of periodic acid which accordingly are lost for further use in the process. In view of the high cost of iodine and its compounds, this represents a serious economic loss which can spell out the difference between a procedure for the production of dialdehyde cellulose which is commercially successful, and one which is not.